The invention relates to resistance spot welding machines. More particularly, the invention is directed to a motor-driven resistance spot welding machine including a robot welding gun.
A conventional motor-driven C-type spot welding gun for a robot is designed as follows. An electrode force applying mechanism in which a ball-bearing leadscrew is set into a nut that is interlocked with the output shaft of an electric motor through transmission means, is secured to one end of a C-type gun arm, and a pair of electrode portions confronting each other are disposed on an end of the ball-bearing leadscrew and the other end of the gun arm, respectively. By causing the electric motor to rotate the nut, the ball-bearing leadscrew is driven linearly to thereby apply electrode force (see Japanese Utility Model Publication No. Hei. 7-31899).
Further, the following electrode force applying methods have heretofore been proposed to drive a conventional C-type motor-driven resistance spot welding gun. One method of applying electrode force involves the steps of: causing a pinion rotated by an electric motor to mesh with a rack having an electrode portion disposed thereon; and causing the electric motor to rotate the pinion with the rack held in a bearing fixed at the side of a gun arm, so that the rack makes a linear motion. Another method involves the step of causing a shaft brought into slidable contact with a cam driven by an electric motor to move linearly, so that an electrode portion disposed on an end of the shaft is driven to apply electrode force (see Japanese Patent Unexamined Publication No. Hei. 7-124753).
However, the following problems have heretofore been addressed in these conventional arts.
First, the former apparatus employing a ball-bearing leadscrew and a nut has suffered from the following problems.
(1) The rotating section is susceptible to wear due to entrance of welding splashes (tiny iron particles). When the tiny particles scattering at the time of welding enter into the ball-bearing leadscrew, these tiny iron particles circulate deep into the leadscrew together with the balls, which in turn causes a fatal damage such as galling. Therefore, adequate preventing measures against entrance of the tiny iron particles must be taken. However, it is practically impossible to perfectly prevent the entrance of the tiny iron particles for a long term.
(2) The electrode reciprocates between an open position and a pressing position to a workpiece at each welding. That is, the electrode repeats the cycle of stop and movement. At each cycle of stop and movement, a motor must be rotated at from 0 rpm to 1000-3000 rpm within a period as short as 0.05 to 0.1 sec. That is, the rotation of the motor must be speeded up in a short time. However, in order to meet this requirement, the capacity of the motor must be increased since the moment of inertia (GD.sup.2) of the ball-bearing leadscrew and the nut are larger by one order of digit than that of the reduction gear of the invention.
(3) Further, grease is splashed due to high-speed rotation of the ball-bearing leadscrew, which in turn makes it practically difficult to maintain sufficient lubrication continuously. Therefore, it is also difficult to allow the ball-bearing leadscrew to complete a design life thereof.
(4) The length of the gun main body is increased in association with the distance of the ball-bearing leadscrew moved, which in turn causes the gun-main body to interfere with the robot wrist. In order to avoid the interference, the surface of the robot onto which the gun bracket is mounted must be extended backward. The possibility of downsizing is disadvantageously restricted.
(5) The length of a ball-bearing leadscrew must be changed so as to correspond to the specified maximum stroke of guns. In addition, the ball-bearing leadscrews, being special precision parts, are not suitable for mass production, so that the cost and appointed date of delivery of the leadscrews impair the productivity of welding guns.
The latter apparatus employing a rack and a pinion has suffered from the following problems.
(1) Welding splashes tend to deposit on the stepped portions of teeth. When welding splashes enter into the meshed portion between the rack and the pinion, excessive load is applied to the toothed surfaces, which results in a fatal damage to the apparatus.
(2) It is difficult to take effective welding splash preventing measures for overcoming the aforementioned problem. Since the teeth of the rack project out by a distance exceeding a stroke in each of both sides from the axial center of the pinion, a cover must be put over the entire part of the rack and the pinion. As a result, the gun becomes large in structure as a whole. In addition, even if a dust seal or a metal scraper is employed as a splash preventing measure, such measure is technically difficult for the teeth of the rack which are of stepped shape.
(3) To prevent the teeth of the rack and pinion from wearing quickly, grease must be continuously supplied to the tooth surfaces. The teeth of the rack move a distance exceeding a stroke in each of both sides from the shaft center of the pinion. It is difficult to design and fabricate a cover that can close the rack perfectly-while allowing the rack to breathe inside the cover with the rack moving to and fro.
(4) The rack needs a guide mechanism so that the center of the electrode portion attached to the end of the rack is not displaced. The structure of such guide mechanism becomes complicated if the rack has a rectangular cross section, and a separate guide rod must be provided if the rack has a round cross section. Therefore, considerations must be given to avoiding interference between the guide mechanism and the workpiece.
(5) It is a fatal restriction that the maximum open stroke is only about several millimeters in a mechanism involving a cam and a shaft. A maximum open stroke of ten-odd centimeters is generally required for a spot welding gun. Therefore, such mechanism is not applicable to the spot welding gun.
As described above, both the rack and pinion mechanism and the conventional cam and shaft mechanism are not suitable as pressing mechanism of the welding gun.
Generally speaking, conventional welding guns of this type require a dedicated gun bracket that allows a gun to be mounted on a robot arm. Therefore, a newly designed gun according to customer's specification, requires a large number of engineering and manufacturing hours, and suffers from the problem of difficulty in unitized standard parts for each function. As a result, there has been a fact that the same part can not be used for welding guns because of slight difference in the specification though the functions of the welding guns are quite identical to each other.